1. Field of the Invention
This invention relates to an electric power steering apparatus that detects steering torque transmitted by a steering shaft by a torque sensor, and provides steering assistance power in response to the torque detected by an electric motor, and a method for producing the same.
2. Description of the Related Art
It is known that there is great demand for enhanced fuel economy in many fields, especially automobiles, from the viewpoint of environmental protection. As a means for achieving this, use of a power steering apparatus powered by an electric motor is being considered, and some electric power steering apparatuses have already been put into practical use.
The electric power steering apparatus is disclosed in, for example, Japan Patent Publication (koukai) No. Heisei 2-132336 (incorporated herein by reference). According to that publication, an input shaft (first shaft) and an output shaft (second shaft) are connected by an elastic member (torsion bar). A first induction member and a second induction member, corresponding to a first detecting member and a second detecting member, are mounted at the end of the input shaft and the output shaft for rotating therewith so that the first induction member and the second induction member are in opposition to one another. The induction members are discs or concentric cylindrical elements made of a conductive material, each member having sector-shaped cut-out portions. The sector-shaped cut-out portions of the two induction members are aligned for a predetermined starting torque. Detecting coils are mounted on both sides of the induction members and generate a high frequency oscillating magnetic field. The magnetic field induces eddy currents in the induction members. The eddy currents in the second induction member increase as the conduction material of the second member is exposed to the detecting coil through the cut-out portions of the first induction member. Increases in the induced eddy currents cause decreases in inductivity of the detecting coil which, when measured, constitute a measure of the transmitted torque. One of the detecting coils is used for detecting torque generated between the input shaft and the output shaft, the other detecting coil is used for temperature compensation. Therefore, it is preferable that at least two detecting coils are provided for the electric power steering apparatus. Each detecting coil is stably fixed to both sides of the induction members, by being fixed to a housing.
In the known electric power steering torque sensor described above, the induction members are mounted at the input shaft and the output shaft and the detecting coils are fixed to the housing so that the induction members are sandwiched between the detecting coils. This design is complicated to assemble. In light of this problem, an electric power steering torque sensor that can be easily installed on the steering column during vehicle assembly is desired.
According to an embodiment of the present invention an improved electric power steering apparatus is provided. The electric power steering apparatus comprises, a first shaft, a second shaft, an elastic member connected to the first shaft at an end of the elastic member and connected to the second shaft at the other end of the elastic member, an electric motor for generating steering assistance power, the electric motor having an output shaft, a worm shaft connected to the output shaft of the electric motor, a worm wheel provided on the second shaft, the worm wheel meshing with the worm shaft so that the steering assistance power is transmitted from the electric motor to the second shaft, a first detecting member provided to the second shaft, the first detecting member being made of magnetic material, the first detecting member having a plurality of radial notched portions that define a first set of slots, the first set of slots provided on at least one side of the first detecting member so that of the slots are equally space radially about the first detecting member, a pair of second detecting members provided on both sides of the first detecting member so that the first detecting member is sandwiched axially between the two second detecting members, the second detecting member being made of a conductive and non-magnetic material, the second detecting member having a plurality of radial notched portions that define a second set of slots equally spaced radially about the second detecting members, a pair of detecting coils axially disposed on either side of the first detecting member and the second detecting members so that the set of the first detecting member and the second detecting members are sandwiched between the detecting coils, the detecting coils detecting a variation of torque generated between the first shaft and the second shaft, wherein the variation of torque is detected by measuring the variation in impedance in the detecting coils which varies according to the amount of over lap between the first slots and the second slots, a pair of yokes made of magnetic material, each of the yokes accommodating the detecting coil so that the yoke surrounds the detecting coil, a housing accommodating the first detecting member, the second detecting members, the detecting coils, and the yokes, wherein the yokes are fixed to the housing, wherein the largest outer diameter of the first shaft is smaller than each of an inner diameter of the first detecting member, the second detecting member and the yokes.
According to another aspect of the present invention, a method for assembling an electric power steering apparatus is provided. The method of assembling an electric power steering apparatus comprises the steps of connecting a first shaft to an end of an elastic member, connecting a second shaft to the other end of the elastic member, the second shaft having a worm wheel, thereafter assembling an electric motor for generating steering assistance power, the electric motor having an output shaft and a worm shaft connected to the output shaft of the electric motor, the worm wheel meshing with the worm shaft so that steering assistance power is transmitted from the electric motor to the second shaft, installing a torque sensor on the first shaft and the second shaft, wherein the torque sensor comprises, a first detecting member provided on the second shaft, the first detecting member being made of magnetic material and having a plurality of radial notched portions that define first slots, the first slots provided on at least one side of the first detecting member so that the first slots are spaced around the first detecting member, a pair of second detecting members provided on both sides of the first detecting member so that the first detecting member is axially sandwiched between the second detecting members, the second detecting member being made of conductive and non-magnetic material and having a plurality of radial notched portions that defines second slots so the second slots are spaced around the second detecting members, a pair of detecting coils axially provided on both sides of the first detecting member and the second detecting member so that the first detecting member and the second detecting members are sandwiched between the detecting coils, the detecting coils being capable of detecting a variation of torque generated between the first shaft and the second shaft based on a change in impedance of the detecting coils wherein the change in impedance varies according to a variation of an over lap between the first slots and the second slots, a pair of yokes made of magnetic material, each of the yokes accommodating one of the detecting coils so that each yoke surrounds the one of the detecting coil, and thereafter attaching a housing accommodating the first detecting member, the second detecting members, the detecting coils, and the yokes, wherein the yokes are fixed to the housing, wherein the largest outer diameter of the first shaft is smaller than each of inner diameters of the first detecting member, the second detecting member and the yokes.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.